This invention concerns a method for the surface treatment of zinc plated steel sheets in which a composite film of chromate and silica which has excellent corrosion resistance, paint binding properties and film uniformity is formed on the surface of galvanized steel such as molten zinc plated steel, zinc electroplated steel, zonc alloy electroplated steel and alloy zinc plated steel.
Conventional methods of treatment in which Cr.sup.6+, Cr.sup.3+ -silica based treatment liquors are used have been disclosed in Japanese Pat. No. 42-14050 (1967), Japanese Pat. No. 45-38891 (1970), Japanese Patent Kokai No. 52-17340 (1977), and Japanese Patent Kokai No. 52-17341 (1977) and various metods of coating have been used. However, when the extent of film formation is increased to improve the corrosion resistance, the amount of silica attached is also increased along with the compounding ratio Cr.sup.6+, Cr.sup.3+ / silica in the treatment liquor and so the degree of adhesion between the metal which is being treated and the film which is formed is reduced. Thus, there is a tendency for the performance to become poor in respect of painting properties.
Furthermore, with these methods of coating, the concentration of the treatment liquor must be varied in order to control the extent of film formation or a device must be provided for changing the shape of the coating roll or the roll pressure and so it is diffcult to change the extent of film formation quickly and to achieve the correct control. There is a further difficulty in that here are other problems with these methods of coating such as a lack of uniformity of the film.
Methods of cathodic electrolysis treatment in Cr.sup.6+ treatment liquors have been indicated in Japanese Pat. No. 47-44417 (1972) (a method in which zinc plated steel sheets are subjected to a cathodic electrolysis treatment in a CrO.sub.3 -H.sub.2 SO.sub.4 treatment liquor) and Japanese Pat. No. 48-43019 (1973) (a method in which a cathodic electrolysis treatment is carried out in a CrO.sub.3 - heavy metal ion based treatment liquor). In general, the films which are formed by means of a cathodic electrolysis treatment in a Cr.sup.6+ treatment liquor have inadequate corrosion resistance but they are said to have excellent painting properties. However, the painting properties have not always been satisfactory in industrial terms.
Furthermore, the following problems arise with these conventional cathodic electrolysis treatments in Cr.sup.6+ based treatment liquors in connection with the achievement of a surface treatment which is stable industrially. Thus, when zinc plated steel sheet is treated continuously, zinc ions are dissolved into the liquor and the Cr.sup.3+ ions which are produced by a reduction reaction during the cathodic electrolysis are precipitated due to the increase in the pH of the treatment liquor so the stability of the treatment liquor is poor. Moreover, there is a change in the external appearance of the film which is formed and problems also arise with worsening corrosion resistance.
This invention is intended to provide an improvment in respect to the difficulties experienced with controlling the extent of film formation and the poorer painting properties observed when the extent of film formation is increased, the disadvantages of the conventional Cr.sup.6+, Cr.sup.3+ - silica based treatment liquor coating methods, to improve upon the variations in external appearance of the film and the stability of the treatment liquor and the poorer corrosion resistance which arise during continuous operation, the disadvantages of the conventional cathodic electrolysis treatment methods, and to provide for the industrially stable formation of films which have superior external appearance, corrosion resistance, and painting properties on the surfaces of zinc plated steel sheets.